1. Field of the Invention
The present invention relates to an image forming apparatus wherein a coil generates a high-frequency magnetic field, the high-frequency magnetic field is applied to a heat generation member to produce an eddy current, and the heat which the heat generation member generates due to the eddy current loss is used for fixing a developer image onto a recording medium.
2. Description of the Related Art
A fixing unit adapted for an image forming apparatus using digital technology, namely, an electronic copying machine, is in practical use. The fixing unit is provided with a heating roller, and a pressing roller which is in contact with the heating roller. A sheet is fed, sandwiched between these rollers. Meanwhile, the heat from the heating roller fixes a developer image onto the sheet.
An induction heating device is an example of a heat source of the heating roller. The induction heating device comprises a coil provided inside the heating roller, and a high-frequency generation circuit that supplies a high-frequency current to the coil.
The high-frequency generation circuit includes a rectifying circuit for rectifying a voltage provided by an AC voltage source, and a switching circuit for converting the output voltage (D.C. voltage) of the rectifying circuit into a high-frequency wave of a predetermined frequency. The coil described above is connected to the output terminal of the high-frequency generation circuit (i.e., to the output terminal of the switching circuit).
When the high-frequency generation circuit operates, the coil is supplied with a high-frequency current and thus generates a high-frequency magnetic field. This high-frequency magnetic field is applied to the heating roller, producing an eddy current in the heating roller. The heating roller generates heat due to the eddy current loss, and the heat serves to fix a developer image onto a sheet.
In this type of apparatus, the heating roller is kept at a predetermined temperature (a temperature that enables a fixing operation) by the temperature control of the main body of the copying machine.
In the apparatus, a driving signal with which to drive the high-frequency generation circuit is monitored in relation to time. Based on this monitoring, the heating roller is prevented from being heated to more than a predetermined temperature, and the fixing unit and the copying machine are thus prevented from igniting.
The monitoring based on time is executed at predetermined intervals. That is, it is executed without reference to changes in the environments in which the apparatus is installed, such as a change in temperature. Hence, the time-based monitoring of a driving signal, with which to drive the high-frequency circuit, is not properly executed in accordance with the conditions.
The present invention relates to an image forming apparatus comprising a fixing unit which is provided with an induction heating apparatus wherein an eddy current is generated in a heating roller by the generation of a high-frequency magnetic field from a coil and a heating roller generates heat due to the eddy current loss, and which uses the heat generated by the heating roller to fix a developer image onto a recording medium. The object of the invention is to enable the image forming apparatus to execute processing in accordance with the environments.
The image forming apparatus of the present invention is of a type including a fixing unit that fixes a developer image onto a recording medium by utilization of the heat generation by the heating roller. The image forming apparatus comprises: a sensing section that senses a temperature of the heating roller; an output section that outputs a driving signal on the basis of the temperature sensed by the sensing section; an induction heating device including: a coil that is received inside the heating roller; a high-frequency generation circuit that supplies a high-frequency current to the coil; a control element that outputs a control signal to the high-frequency generation circuit on the basis of the driving signal output from the output section and that monitors a successive output time of the driving signal from the output section at different points of time corresponding to different environments; and processing means for supplying the high-frequency current provided by the high-frequency generation circuit to the coil based on the control signal from the control element, the induction heating device producing an eddy current in the heating roller by causing the coil to generate a high-frequency magnetic field, and causing the heating roller to generate heat based on an eddy current loss.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.